The present invention relates to methods of obtaining distributed measurements using optical fibres, and apparatus therefor.
Techniques for using optical fibres to obtain distributed measurements of various parameters, such as optical time domain reflectometry and distributed temperature sensing, are well known. The underlying principle of these techniques is that light is injected into an end of the fibre, and undergoes scattering at all points along the length of the fibre. The amount and/or wavelength of the scattered light is affected by various parameters, such as temperature. Some of this light is backscattered to the fibre end, and the time at which it arrives at the fibre end is related to the position along the fibre at which it was scattered, owing to the constant speed of light within the fibre. Thus, detection of the backscattered light over time gives a representative profile of the parameter of interest over the length of the fibre.
Temperature can be detected by looking at backscattered light arising from the inelastic scattering process known as Raman scattering. This produces a pair of spectral bands shifted one to either side of the wavelength of the original injected light. The longer wavelength band is referred to as the Stokes component, and the shorter wavelength band as the anti-Stokes component. The amplitude of these components is temperature-dependent, so a distributed fibre system arranged to detect one or both of these components can be used as a temperature sensor [1].
Two arrangements may be considered for obtaining a temperature measurement. In either case, the anti-Stokes component is measured. This can be compared with either the Stokes component, or with backscattered light that has undergone elastic scattering (Rayleigh scattering) and hence has the original wavelength. In each situation, though, light at two different wavelengths is detected.
Light propagating in optical fibres experiences loss; this can vary with wavelength. This presents a problem in distributed temperature sensing (and other sensor arrangements that detect more than one wavelength), because the two detected components may undergo different amounts of loss in propagating from the scattering site to the fibre end. Thus a ratio obtained by comparing the two detected components is dependent not only on the amount of Raman scattering (the desired information), but also on the difference in loss suffered by the backscattered light on its return to the fibre end. The ratio can thus be distorted, giving an inaccurate measurement.
Unfortunately, it is very difficult to determine this differential loss and thus separate the effects of propagation loss from those of the temperature profile being measured.
One approach that has been taken to address this issue is to allow an estimate of the loss to be entered into the measurement system. However, such estimates tend to assume that the loss is constant or piece-wise constant along the fibre length. This is typically untrue, owing to nonuniformities in the fibre, such as bends and splices. Also, no account is taken of changes which may occur over time as the fibre degrades.
A more successful technique is to use a double-ended measurement method, in which the measurement is repeated from the other end of the fibre [2]. The additional information conveyed by the second measurement is sufficient to allow the effects of temperature to be separated from those of differential loss, because the temperature tends to appear the same regardless of the measurement direction, whereas the loss appears in an opposite sense which viewed from the other fibre end.
However, double-ended systems have a major disadvantage over single ended systems in which all measurements are made at only one end of the fibre, in that severe restrictions are imposed on installation of the fibre at the measurement region of interest. Instrumentation is required at both ends of the fibre, which is more costly and complex. In circumstances where the remote end of the fibre is inaccessible, it is necessary to install the fibre as a loop, so that both ends are in the same location. This can be awkward to achieve, and also doubles the length of the fibre required in situations where no additional information can be gleaned from the return part of the fibre, thus increasing overall propagation losses.
A further consideration is that of the maximum power that can be launched into the fibre. Ideally, a large amount of power should be launched, to give large return signals. This increases accuracy by improving the signal to noise ratio, and also reduces measurement times. However, in the case where the anti-Stokes signal is compared to the Stokes signal, high powers can distort the ratio by causing unwanted nonlinear effects. At low powers, the Raman scattering is spontaneous. If the injected optical power exceeds a particular threshold, however, stimulated Raman scattering will occur to a degree which depends on the incident light intensity. The stimulated scattering is nonlinear and converts power from the incident light to the Stokes component and hence alters the Stokes/anti-Stokes ratio. To avoid this it is therefore necessary to operate at a power level at which stimulated scattering converts no more than an acceptable fraction of the light.
A technique which addresses both the differential loss problem and the stimulated Raman scattering problem has been proposed [3]. The nonlinear effects are addressed by measuring the anti-Stokes light and the Rayleigh light to obtain the desired ratio signal. The Stokes component is not considered, so cannot distort the output. A first optical source is used to generate the light to give these two signals. In addition, a second optical source is provided which emits at the anti-Stokes wavelength of the first source, and a Rayleigh measurement is obtained at this wavelength. Thus three signals are measured, with the Rayleigh measurements being independent of the temperature but including the loss at the two wavelengths. To obtain a final output which is independent of any differential loss at the two wavelengths, the Raman measurement is normalised to the geometric mean of the two Rayleigh measurements.
However, this method does not entirely account for nonlinear distortion, although it is more robust than the Raman anti-Stokes/Stokes ratio method. If the threshold for stimulated Raman scattering is exceeded, not only is the Stokes component distorted, but the original injected light is depleted by the power transfer to the Stokes wavelength. This reduces the amount of light undergoing Rayleigh scattering, so that the detected Rayleigh signal is reduced, which in turn distorts the measured temperature profile.
As mentioned above, the nonlinear effects can be avoided by operating at low optical power levels, but this is not desirable. In particular, the low power reduces the maximum length of fibre that can be used before the total fibre loss becomes too high.
Therefore, there is a need for an improved distributed sensing method.